SEOUL, April 25 (Reuters) - North Korea represents a threat not just to Asia but to the United States, U.S. President Barack Obama said on Friday, as he and South Korea's President Park Geun-hye warned they would respond firmly to any "provocations".

In March, the North warned it would not rule out a "new form" of atomic test to boost its nuclear deterrent after the U.N. Security Council condemned Pyongyang's launch of a mid-range ballistic missile into the sea east of the Korean peninsula.

Recent satellite data shows continued work at the nuclear test site in North Korea, although experts analysing the data say that preparations do not appear to have progressed far enough for an imminent test.

"When North Korea is threatening further provocations and publicly discussing the possibility of a further nuclear test, President Obama's visit to South Korea will send a firm message that North Korea's provocations will not be tolerated," Park told a joint press conference.

The two presidents were speaking after a summit in Seoul, the second stop of a four-nation Asia tour for Obama.

Obama hopes his tour will reassure allies in the region that Washington will keep its promise of rebalancing some of its resources toward the fast-growing countries of East Asia. He has had to strike a balance between showing the United States will be a counterweight to China without offending Beijing, which worries Washington wants to contain its growth and influence.

CHINESE INFLUENCE

Obama said he hoped China would use its influence to rein in its North Korean ally. Beijing called again on Friday for a resumption of stalled talks between North Korea, itself, the United States, South Korea and Russia.

Park said the process - known as the six-party talks - would be rendered "useless" by a fourth nuclear test.

The two leaders said that North Korea's recent bluster provided grounds for delaying a scheduled transfer of operational control over South Korean troops in 2015 to Seoul from the U.S.-led United Nations Command.

North Korea has been under U.N. sanctions since its first nuclear test in 2006, banning it from conducting atomic and missile tests, barring U.N. member states from weapons trade with Pyongyang and financial transactions that facilitate them.

Military experts say the North would lose any conventional war with South Korea and the United States and is seeking to develop nuclear weapons as a deterrent.

It has also used the nuclear programme to attempt to wring concessions and aid from the United States. Washington has said it would be open to talks if Pyongyang abandoned its nuclear programme.

North Korea describes its nuclear weapons as a "treasured sword" that it will never abandon.

Obama stressed deep U.S. ties to South Korea throughout his visit, offering condolences to Park and the Korean public for the ferry sinking tragedy that killed hundreds of people, many of them children.

Obama also laid a wreath at a memorial for U.S. troops who died in the Korean War 1950-1953.

Separately, North Korea said on Friday it had detained a 24-year-old American tourist earlier this month for what it called "a gross violation of its legal order". The official KCNA news agency said Matthew Todd had torn his tourist visa to shreds. (Reporting by Ju-min Park; Writing by James Pearson; Editing by Alex Richardson and Gareth Jones)